Currently, distance measurements include contact distance measurements and non-contact distance measurements. Wherein the contact distance measurements are conventional, and an example of which is called coordinate measuring machine (CMM). Although the contact distance measurements have been very precise, as the contact distance measurement should contact the object, probes of the contact distance measurement instruments may damage the object. Therefore, the contact distance measurement instruments are not suitable for expensive objects.
Relative to the conventional distance measurement instruments, the non-contact distance measurement instruments can have an operation frequency of several millions, therefore are widely used in a variety of fields. The non-contact distance measurement instruments include active non-contact distance measurement instruments and passive non-contact distance measurement instruments. A typical active non-contact distance measurement instrument usually projects an energy wave to an object and computes a distance between the object and a reference point through reflection of the energy wave. Common energy waves include visible light, high power light beam, ultrasonic wave and X-ray.